Due to its advantageous electrical and chemical properties and its substantial natural abundance, the elemental semiconductor silicon is probably the most studied substance on Earth and is the dominant material used in the manufacture of micro/nano-electronic devices, micro/nano-electro-mechanical devices and systems (MEMS/NEMS), photovoltaic devices, and other related technologies.
Notwithstanding its many advantages and the unrivalled expertise in its processing and properties that has developed over more than 50 years, other semiconductors can have advantages over silicon for some applications. For example, the higher carrier mobilities, wide bandgaps, and direct bandgaps of some compound semiconductors make such materials preferable for use in extremely high speed specialised micro/nano-electronic devices and light emitting devices such as solid state lasers and light emitting diodes (LEDs).
One compound semiconductor of great interest is silicon carbide (SiC), an extremely hard ceramic material having a wide bandgap, high thermal conductivity, a low thermal expansion coefficient, the ability to conduct high current densities, and high chemical resistance. Moreover, silicon carbide is the only semiconductor other than silicon itself that reacts with oxygen to form a device quality insulating oxide of SiO2, thus greatly facilitating the fabrication of semiconductor devices. Other compound semiconductors generally require a device quality dielectric layer to be deposited, a substantial shortcoming that it one of the reasons why such materials have not been widely adopted. Finally, SiC can be doped to form n-type and p-type conducting regions by introducing impurities (e.g., nitrogen and aluminium). These properties have enabled SiC to become an important material for the manufacture of high temperature and high power electronic devices, and also indicate its further potential for the development of discrete devices, integrated circuits and MEMS/NEMS devices with superior properties to those of devices made from silicon, including high chemical resistance and radiation hardness, high temperature operation, high power capability, high speed and high efficiency operation.
Although silicon carbide wafers are commercially available, they are extremely expensive and are only available in small wafer sizes (e.g., 2 inch diameter @ A$1,500, and 4 inch @ A$5,000), due to the difficulty of growing SiC with sufficient low defect densities for device applications. Boules of single-crystal SiC are formed at temperatures around 2200° C., which poses extreme challenges for processing equipment. For comparison, boules of single-crystal silicon are formed at substantially lower temperatures around 1410° C. 150 mm (≈6 inch) diameter prime silicon wafers cost only about A$30 per wafer.
Silicon carbide has an extremely large number of different polytypes, with the 4H and 6H being the most common polytypes used to form bulk wafers. However, thin layers of the 3C polytype can be epitaxially grown on Si wafers, thereby allowing highly evolved and well characterized silicon chip technology to be combined with the enabling properties of silicon carbide to provide enhanced capabilities on a cost-effective platform that can be rapidly introduced to the market. Additionally, an epitaxial film of silicon carbide on a silicon wafer can also be used as a buffer layer for the subsequent deposition of one or more layers of other single-crystal semiconductors having compatible lattice constants to enable the fabrication of devices from these materials at relatively low cost and using silicon wafer processing equipment.
Device manufacturing in SiC generally requires high quality single-crystal SiC with low defect and impurity densities. Where the SiC is in the form of a layer epitaxially grown on a single-crystal silicon substrate, the large lattice mismatch between silicon and silicon carbide prevents a perfect crystal transition from silicon to silicon carbide. Additionally, impurities introduced into the SiC film during deposition can degrade its quality. Moreover, such impurities can preferentially precipitate or otherwise be trapped at defects within the SiC crystal lattice and/or may even cause the formation of anti-phase boundaries, stacking faults and/or dislocations, thereby further degrading the electronic and optical properties of the SiC film.
Epitaxial deposition of compound semiconductors by low pressure chemical vapour deposition (LPCVD) is generally a slow process. Due to the limited surface mobilities of the atomic species deposited onto the growth surface, long deposition times are required to enable those species to move to the lowest energy atomic locations and thus form high quality single crystal materials with low defect densities. However, long processing times also increase the potential for contamination. Undesirable impurities such as water vapour, oxygen, gaseous oxide and nitrogen typically have very high sticking coefficients and can easy be adsorbed onto or react with the surface of the deposited semiconductor film, thus contaminating the film. By contrast, gaseous deposition precursors such as silane (in the case of SiC deposition) can have very low sticking coefficients (depending upon the process conditions), whereby only fractions of a percent of surface collisions result in the desired atomic species (e.g., silicon in the case of silane) remaining on the surface. Hence it is extremely important to ensure that the levels of contaminants are very low.
In the scientific literature dating back decades, many attempts to deposit device quality silicon carbide on silicon have been made, with very limited success overall. In particular, attempts to develop processes and apparatus for the commercial scale production of 3C silicon carbide on silicon wafers have been unsuccessful.
For example, one such attempt is described in Nagasawa and Yagi, 3C-SiC single crystal films grown on Si Substrates, Phys. Stat. Sol. (b) 202, 335 1997 (hereinafter “Nagasawa”). Nagasawa describes a SiC deposition system having a hot walled silica reaction tube. However, at low pressures and high temperature, silica is not leak tight and hence allows contaminants to pass through the tube wall. Additionally, due to the pumping system and vacuum integrity of the system (leak tightness), the operational window and base pressure of the system are limited. The lack of leak integrity is apparent from the graphs showing partial pressures during processing. Even at these high flows and pressures evidence of water vapour (17 and 18 atomic mass units (amu)) and nitrogen (28 amu) are evident. The process described in Nagasawa uses a first gas precursor, pumps out that gas, and then flows a second precursor to form silicon carbide. However, the inventor believes that the exposure of the deposition surface to contaminants throughout the process is likely to limit the deposited film quality that the Nagasawa system is able to produce.
Despite the passage of more than a decade since the publication of Nagasawa's paper, little progress appears to have been made in creating a deposition system capable of producing device quality 3C silicon carbide films on Si, even in small wafer sizes.
Indeed, systems for depositing device quality silicon carbide films are not readily available and a settled design has not been achieved to date because a commercially viable process for depositing silicon carbide on silicon has not yet been established.
U.S. Pat. No. 5,861,346 discloses a method of forming a silicon carbide layer on a silicon substrate using a C60 precursor.
It is desired to provide a chemical vapour deposition system and process that alleviate one or more difficulties of the prior art, or that at least provide a useful alternative.